This invention relates generally to a vehicle automatic transmission and more particularly to a hydraulic pressure regulating system for a vehicle automatic transmission which prevents damage due to high internal fluid pressures. The line pressure applied to the hydraulic control system of an automatic transmission mounted on a vehicle is generated by an oil pump driven by the engine of the vehicle. The pressure of the fluid discharged from the oil pump is regulated to a line pressure by a pressure regulator valve. The line pressure is regulated to a level commensurate with the level of torque to be transmitted so as to reduce the power consumed by the oil pump. Conventionally, the pressure of the fluid discharged from the oil pump becomes excessively high when the engine operates at a high revolving rate during a high speed cruising state of the vehicle. In order to regulate this line pressure by means of a pressure regulator valve, pressure signals are provided to the pressure regulator valve to control the rate of draining the pressurized fluid supplied by the oil pump. These pressure signals may be, for example, the governor pressure, throttle pressure and line pressure which vary according to the running condition of the vehicle, such as transmitted torque. A modulator valve is provided in the input oil passage to the pressure regulator valve to control the input oil pressure when necessary.
In a hydraulic control circuit employing an oil pressure (line pressure) regulating device of the type as described above, the line pressure is liable to rise excessively and cause problems such as damage to sealing members of the automatic transmission when the modulator valve malfunctions, for example, due to a sticking valve. In order to prevent such an excessive increase in the line pressure, a conventional pressure regulating device comprises a relief valve of a large flow capacity in the output passage of the device. Provision of such a relief valve of a large capacity disadvantageously affects efforts to design a compact hydraulic control circuit.
What is needed is a hydraulic pressure regulating system for a vehicle automatic transmission which is compact and provides relief protection from excessively high oil pressures within the transmission and regulating system.